Journal of Human Kinetics volume 40/2014, 93-102 DOI: 10.2478/hukin-2014-0011
Section II- Exercise Physiology & Sports Medicine
93
Norms for an Isometric Muscle Endurance Test
by
Sarah L. Strand , John Hjelm , Todd C. Shoepe1, Marie A. Fajardo1
1
2
Musculoskeletal performance assessment is critical in the analysis of physical training programs in order to
prioritize goals for decreasing injury risk and focusing performance goals. Abdominal endurance as part of this analysis
is often assessed with techniques that have validity that has been debated in literature. The purpose of this study was to
develop normative sex- and athlete-specific percentiles for a trunk stabilization and muscular endurance by using a
prone forearm plank test in college-aged students. A second purpose of this study was to investigate the effect of
habitual physical activity and the reason for test termination. There were 471 participants (means SE; males: n = 194,
age 20.4 0.2 years, body height 179.4 0.5 cm, body mass 81.1 1.2 kg; females: n = 277, age 20.2 0.2 years,
body height 165.7 0.4 cm, body mass 63.9 0.7 kg) who performed this test to volitional or technique failure. Males
produced significantly higher test durations than females (means SD; 124 72 seconds vs. 83 63 seconds) and
athletes produced significantly longer test durations than non-athletes (123 69 s vs. 83 63 s) but no interaction
effects were seen in the variables of sex and athletic status. The activity level was found to have a threshold of influence
(>3 times/week) on abdominal endurance that is dose-specific where greater than 5 times/week showed the greatest
influence. The fatigue of the abdominals was the termination reason producing the lowest test duration and there was
no sex effect on reason for test termination. These normative percentiles for abdominal endurance suggest that the
abdominal plank test can now be used as an alternative to other abdominal assessments in college students, but further
investigation is warranted prior to confirmation and generalization to other populations.
Key words: Fitness, testing, exercise evaluation, muscular endurance.
Introduction
The abdominal “core” has become an area of
intense scrutiny for researchers, practitioners and
exercise participants in recent years. The core is the
foundation by which all appendicular movement
relies and it includes the ability to dynamically
stabilize the spine, hips, pelvis, proximal lower
limb, and abdominal structures (Akuthota, 2004;
Faries and Greenwood, 2007; Kibler et al., 2006;
Putnam, 1993). All musculature traversing or
supporting these areas is involved with the core
including the transverse abdominis, internal and
external obliques, and rectus abdominis for the
abdominal muscles. In addition, the latissimus
dorsi, pectoralis major, hamstrings, quadriceps,
iliopsoas, upper and lower trapezius, hip rotators,
1
2
and glutei make up the remaining muscles of the
core (Kibler et al., 2006; Moraes et al., 2009). The
core is an important aspect because it is necessary
not only in sports performance but in activities of
daily living by gaining stability, improving posture,
enhancing balance and proprioception (Bird et al.,
2006; Hussain et al., 2007; Richardson and Jull, 1995;
Warden et al., 1999). In particular because the core
plays a critical role in power transfer to the
appendicular skeleton, focusing on the core is an
essential part of exercise training. As part of any
pre-screening for injury risk or the prescription of
any exercise program, muscular performance
testing is often included as part of a comprehensive
needs analysis. The two common categories of
‐ , Loyola Marymount University, Los Angeles, CA.
- North Park University, Chicago, IL.
Authors submitted their contribution of the article to the editorial board.
Accepted for printing in Journal of Human Kinetics vol. 40/2014 on March 2014.
94
muscular assessment include strength and
endurance testing where muscular endurance is
defined as the ability to sustain a given level of
force production over time while muscular strength
is defined as the maximum torque exerted by a
muscle or muscle group (Joynt et al., 1993; Lieber,
2002). Moreover, previous research has suggested
that muscular endurance is functionally more
important to the supportive musculature of the core
than muscular strength, so testing should focus on
endurance (Knudson, 1999).
Sit-ups and curl-ups have long been
prescribed in order to improve strength and due to
the desire to assess performance according to
specificity, they have also become the main ways to
assess abdominal endurance. However, sit-ups and
even curl-ups have been shown to perhaps be less
indicative of endurance and more indicative of
muscular strength or muscular power (Hall et al.,
1992). Sit-ups with the feet restrained, in particular
require hip flexor activation, which greatly aids the
sit-up motion and has been hypothesized to
increase the risk of injury because of the movement
involved in the motion of a sit-up. There are several
concerns with the sit-up in addition to the hip
flexor activation alternating patterns of lumbar
flexion coupled with hyperlordosis that has been
linked with increased pressure on lumbar discs
(Baxter et al., 2003; Jette et al., 1984; Juker et al.,
1998; Liemohm et al., 1988; Mcgill, 1995). In
addition, the administration of sit-up and curl-up
tests have been criticized because they require a
high degree of administrator training and
subjective interpretation of form in order to ensure
test validity and reliability (Andersson et al., 1997;
Knudson, 1999).
Because of the long-term use of sit-up and
curl-up assessments in physical education and
fitness training, there is an abundance of data on
these techniques available. Therefore, much of the
existing literature aimed at assessing muscular
endurance or the core has been produced through
sit-ups and curl-ups, which have resulted in wellestablished normative data to rank each individual
based on their performance. Unfortunately, as
outlined previously, there are number of criticisms
and challenges to the validity, reliability, and
generalizability of these core assessments. As a
result, there has been a search for an abdominal and
trunk stabilization exercise that will effectively
challenge the muscles while minimizing the
Journal of Human Kinetics volume 40/2014
Norms for an Isometric Muscle Endurance Test
hypothesized risk of low-back injury (Childs et al.,
2010). The forearm plank test, also referred to as a
prone bridge, has been theorized to be more
functional because it provides for assessment of
endurance
during
an
activity
requiring
simultaneous activation of the entire anterior
muscular chain (Bliss and Teeple, 2005). Plank tests
recruit anterior core musculature and challenge the
core muscles while specifically targeting the
external oblique and lateral stabilizers and
increased activity of the anterior musculature has
shown improved performance (Aggarwal et al.,
2010; Schellenber et al., 2007; Schellenberg et al.,
2007). The plank test provides an adequate stimulus
for endurance training of the rectus abdominis and
external oblique abdominis (Ekstrom et al., 2007). It
has been shown that the rectus abdominis and
external obliques are important for prevention of
injury and improved athletic performance (Nadler
et al., 2002; Tsai et al., 2004; Watkins et al., 1996).
Therefore, the purpose of this study was to
describe the prone plank test as an alternative
assessment of muscular trunk endurance through
the creation of percentiles for the purpose of
ranking
college-aged
participants
in
the
establishment of data norms separated by sex and
athletic status. We proposed to accomplish this
through a test that was less complicated to
administer, as well as to increase the construct
validity of assessing abdominal endurance, and
lessen the risk of lower-back injury.
Material and Methods
Participants
Following approval of the Human Subject
review boards of Institution North Park University
and Loyola Marymount University, a total of 471
participants (males: n = 194, females: n = 277) were
recruited. Upon completion of oral and written
informed consent, participants were screened for
health restrictions identified by the seven-item
Physical Activity Readiness Questionnaire (Thomas
et al., 1992). Participation was voluntary with no
compensation involved. A total of 109 (23% of the
sample total) NCAA varsity athletes were included
in the analysis compared to 361 who at the time of
the study were not affiliated with a varsity sport at
their respective institution. Participant descriptive
data and anthropometrics are displayed in Table 1.
Where there was no difference in mean age, males
were significantly taller and heavier than females.
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95
by Strand S.L. et al.
When separated by athletic status, the same trends
held for age, body height and mass but not by sex
where athletes were the same age but heavier and
taller on average. The participants also represented
a spectrum of activity levels where the following
levels were reported: never (1%), rarely (13%), 1-2
times/week (26%), 3-5 times/week (41%), and more
than 5 times/week (19%). For those participants that
were college athletes, the activity levels included
their team practice activities and competition.
While it may be assumed that the collegiate athletes
were at the higher end of the activity spectrum,
some were not in-season at the time of testing and
thus, may not have demonstrated peak activity
levels. The sample comprised in this study
represents a heterogeneous group of young adults
affiliated with an institution of higher education
including that, which would be expected from large
urban areas where these institutions reside and in
that the final participant pool demonstrated
diversity in ethnicity and activity level.
Measures
Following a brief technique demonstration,
and detailed instructions, participants were tested
individually. The test procedures were as follows:
the subject assumed the forearm plank position
with elbows in contact with the ground, such that
the humerus formed a perpendicular line to the
horizontal plane, directly beneath the shoulders.
The forearms were in neutral position and hands
were directly in front of the elbows. The participant
assumed a rigid anatomical body position so that
only their forearms and toes supported the body.
This position is characterized by a phalangeal
extension, neutral ankle position, knee and hip
extension, and neutral spinal positions.
Procedures
The participants were instructed to
statically hold this position as long as possible and
verbal cues were provided to the participant briefly
in order to promote form adherence for test validity.
When the subject assumed the proper position, the
investigator started the stopwatch. The test was
terminated when (1) the participant fatigued or
voluntarily stopped the test, (2) the participant
failed to maintain the proper position, (3) the
participant reported ill effects from the test (e.g.
headache, dizziness, pain not associated with
fatigue, etc.), or (4) the investigator noticed signs
indicative of ill effects in the participant from the
test. Participants were provided cues during the test
as technique faltered away from the accepted
position. Tests terminated by the investigator
occurred when two consecutive corrective cues
given to the participant did not result in an
adequate correction in form. At the conclusion of
the test, each participant gave their primary
subjective reason for discontinuation and the
duration time of the test to the nearest tenth of a
second was recorded. To keep consistent with other
types of fatiguing fitness assessments, each subject
only performed the test once.
Table 1
Anthropometric participant data
Total (n=471)
By Sex
male (n=192)
female (n=273)
By Athletic Status
varsity (n=109)
non-varsity (n=361)
Age (years)
20.4  0.1
Body height (cm)
171.0  0.6
Body mass (kg)
70.2  0.8
20.4  0.2
20.2  0.2
179.4  0.5*
165.7  0.4
81.1  1.2*
63.9  0.7
19.9  0.2
20.6  0.2
174.4  0.9#
170.4  0.5
75.9  1.6#
69.3  0.9
All values are given as means  SE.
indicates statistically greater than female.
# indicates statistically greater than non-varsity.
Statistical significance was set at p < 0.05.
*
© Editorial Committee of Journal of Human Kinetics
96
Statistical Analysis
Analysis at baseline examined between-group
differences in age, body height and mass for sex
and athletic status respectively. Potential differences
in mean duration of test by sex or athletic status
were completed with a 2x5 (sex by physical
activity) analysis of variance (ANOVA) whereas a
multivariate ANOVA was used to detect the reason
for termination for the interaction of sex and
athletic status. Activity level effect on test duration
was examined with a one-way ANOVA with five
levels. Percentile rankings were created for sex and
athletic status based on the frequency distribution
of the participants’ test duration. A Chi-Square
analysis was completed to investigate the observed
versus expected reasons for test termination.
Pearson product correlations were completed to
examine potential relationships between the
dependent variable (and thus intended variable of
prediction) of test duration and the independent
variables of both body height and mass by sex and
athletic status. All participant quantitative
assessment was analyzed using SASW for Mac
version 18.0 (IBM; New York, NY) with a statistical
significance set at p < 0.05.
Results
The grand mean (SD) of all participants
for test duration was 100  63 s. Statistical
significance was seen between sexes for the mean
duration of the test in seconds where males were
shown to have test durations 49% higher than
females (124  72 s vs. 83  63 s) and thus normative
percentile rankings were generated for males and
females separately. The resulting median (50th
percentile) was found to be 110 s for males and 72 s
for females. Since it would be a common
assumption that collegiate athletes had higher
activity levels as well as higher strength levels,
compared to the non-athlete participants, the data
for athletes vs. non-athletes were also assessed
separately. There was a significant difference seen
in mean duration of the test according to athletic
status where athletes were found to have test
durations 48% higher than non-athletes (123  69 s
vs. 83  63 s) and thus different percentile rankings
were generated for each of the categorical
definitions of athletic status (varsity athletes versus
non-varsity athletes). A value of 104 s was found to
be the median score for athletes and 83 s for nonathletes. However, examined together as to
Journal of Human Kinetics volume 40/2014
Norms for an Isometric Muscle Endurance Test
potential interactions of sex and athletic status,
there was a very high alpha probability value (p =
0.78) that was therefore not deemed to be
significant. This null finding for an interaction
effect between sex and athletic status did not justify
the further separation of normative percentile
rankings based on sex and athletic status. Table 2
displays the four resulting percentile rankings of
the participants in the study with separation by sex
and again by athletic status.
The effects of activity patterns previous to
the testing session on test duration are displayed in
Figure 1. There was a positive trend for increased
activity to demonstrate longer durations of the
plank test. However, statistical significance was
seen at only the highest two activity levels. More
specifically, those stating that they had been
participating in activity patterns more frequently
than five times per week had significantly longer
test durations than all other conditions.
Furthermore, participants with reported activity
levels of three-to-five times per week produced
longer test durations than those reporting the
lowest two activity categories of “none” and
“rarely” only.
In order of decreasing frequency for all
test participants, the reason for test termination was
as follows: legs (n = 151, 32%), arms (n = 150, 32%),
abdominals (n = 98, 21%), back (n = 36, 6%), posture
(n = 28, 2%), and other (n = 8, 8%). This data
included in and resulting from the Chi-square
analysis for potential differences in reasons
between the sexes are shown in Table 4. There was
no statistical difference seen in any reason category
between the sexes in the primary reason given for
volitional test termination. The resulting totals
presented previously were thus pooled accordingly.
Arithmetically, abdominal failure or discomfort was
the reason for test termination that resulted in the
lowest mean test duration followed by the legs,
posture, arms, and back. However, of these, only
three variables were significantly different. Arm
discomfort or failure was the reason associated with
the longest test duration compared to abdominals
or the legs while the back was significantly longer
than abdominals alone. Despite mean difference
trends there were no additional differences seen
between independent variables for test duration
means.
Correlational
analysis
investigating
relationships between body mass and test duration
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97
by Strand S.L. et al.
yielded statistically small, negative relationships in
all test groups including female non-varsity (r = 0.234; p < 0.001), female varsity (r = -0.321; p = 0.012),
male non-varsity (r = -0.318; p < 0.001), and male
varsity (r = -0.310; p = 0.009). Correlational analysis
investigating relationships between body height
and test duration yielded statistically small,
negative relationships in all test groups with the
exception of female varsity athletes (r = 0.026; p =
0.428). This negative relationship included female
non-varsity (r = -0.135; p < 0.05), male non-varsity (r
= -0.220; p = 0.005), and male varsity (r = -0.239; p =
0.009).
Table 2
Percentiles scores by sex and percentile scores by sport status
Time to Fatigue in the Plank-Test (all values in
seconds)
Percentile Male
Female
Non-Varsity Varsity
(n = 194) (n = 275)
(n = 109)
(n = 361)
10th
62
35
37
59
th
20
79
48
53
66
30th
89
58
62
82
40th
97
63
71
92
50th
110
72
83
104
60th
122
84
94
123
th
70
137
95
106
149
80th
157
108
123
178
90th
201
142
151
200
Percentile
10th
20th
30th
40th
50th
60th
70th
80th
90th
Table 3
Percentiles score by sex and sport status
Time to Fatigue in the Plank-Test (all values in
seconds)
Female
Female
Male
Male
Non-Varsity
Varsity
Non-Varsity
Varsity
(n = 227)
(n = 50)
(n = 134)
(n = 59)
34
45
49
74
47
59
72
84
56
63
83
94
62
74
95
117
70
87
103
125
79
97
115
140
91
110
125
157
103
162
142
183
130
194
189
228
© Editorial Committee of Journal of Human Kinetics
98
Norms for an Isometric Muscle Endurance Test
Table 4
Chi-square results for sex by reason for termination
Reason for Test Termination
Leg
Arm
Ab
Back Posture
Other
Total
Male
Frequency
58
60
44
14
4
14
194
% in sex
29.9%
30.9%
22.9%
7.2%
2.1%
7.2%
100%
Female
Frequency
93
90
54
14
4
22
277
% in sex
33.6%
32.5%
19.5%
5.1%
1.4%
7.9%
100%
Total
Frequency
150
151
98
28
8
36
471
Percent
32.1%
31.8%
20.8%
5.9%
1.7%
7.6%
100%
No differences were seen for the expected frequency of termination reason
across sex (p < 0.05) for any reason.
Figure 1
Activity levels of all participants - All values are given as means  SE.
* indicates statistically greater than all other conditions.
# indicates statistically greater than “Never” and “Rarely”.
No differences were seen for an interaction effect between sex and activity level resulting
in the pooled data shown. Statistical significance was set at p < 0.05.
Discussion
The primary purpose of this research was to
develop normative data for assessing abdominal
endurance with a novel, testing protocol
hypothetically advantageous to existing methods of
abdominal endurance assessment. In this study, 471
Journal of Human Kinetics volume 40/2014
college-aged,
healthy,
males
and
females
representing a diverse ethnic group of urban
participants were examined to produce normative
data on abdominal endurance. With females
demonstrating a statistically lower time to fatigue
than males, percentile rankings for males and
females were produced separately. Also, because
http://www.johk.pl
by Strand S.L. et al.
there was a significantly lower time to fatigue for
non-athletes as compared to athletes, these
percentile rankings were also produced separately
(Table 2). Although there were no significant
differences seen for mean time to fatigue when
evaluated by sex and athletic status, negative
relationships in body size in both sex and athletic
status justified the creation of percentile norms for
these additional four groups: female non-athletes,
female athletes, male non-athletes, and male
athletes (Table 3).
Previous research discusses the use of situp tests and curl-up tests as the most commonly
used ways to assess abdominal endurance. Despite
the widespread use of these tests, it has been
suggested that there are concerns with these tests
both in the objectivity of the tester as well as in the
validity of the role of the participant in the
assessment. As the pioneer in abdominal testing,
sit-up tests in particular have been hypothesized to
induce low back pain likely as a result of elevated
compressive forces as well as an increase in hip
flexor activity especially when the participant
became fatigued (Andersson et al., 1997; Baxter et
al., 2003; Childs et al., 2010; Diener et al., 1995; Jette
et al., 1984; Juker et al., 1998; Liemohm et al., 1988;
Mcgill, 1995; Nachemson and Elfstrom, 1970).
Second to the sit-up test, more recently research has
developed curl-up tests that possibly decrease
lumbar spine stress and hip flexor activity that were
seen with sit-up tests in the evaluation of
abdominal endurance (Juker et al., 1998; Knudson
and Johnston, 1995; Nordin and Frankel, 2001;
Sternlicht and Rugg, 2003). Resulting work reported
that curl-ups were not only able to reduce the
stresses on the lumbar spine and decrease hip flexor
activity, but were able to reproduce similar
abdominal muscle activity to that of sit-ups,
creating a safer method to test abdominal
endurance (Escamilla et al., 2006). Despite some
positive evidence preferring curl-ups to sit-ups,
there remains speculation that the development of
other methods is warranted (Knudson and Johnston,
1995).
One alternative proposed method to sit-ups
and curl-ups in recent years has been the horizontal
plank test which has been suggested to have
promise in being a more accurate assessment
(Schellenberg et al., 2007). The mechanism for this
improved efficacy is because the testing is much
simpler to administer, as there are fewer directions
© Editorial Committee of Journal of Human Kinetics
99
and increased tester objectivity in the ability to
define proper and improper technique. An example
of this is in the difference between static and
dynamic tests. Where the curl-up and sit-up tests
are dynamic assessments requiring movement and
a subjective determination of the appropriateness of
every repetition, the plank test is initiated with a
confirmed starting position and test failure is
determined in part when technique sufficiently
deviates from this established norm. It could be
argued
therefore
that
fewer
subjective
determinations need to be made in the plank test,
which promotes greater validity of the assessment.
Males were found to have significantly
longer test durations on the plank test than a
corresponding group of females. These data
support previous work where sex differences were
seen with the plank position (Schellenberg et al.,
2007). Examining the possibility that physical
activity was a factor in differences for sex for the
variable of test duration revealed that arithmetic
differences were seen for all levels of physical
activity (e.g. never, rarely, 1-2 times/week, etc.).
However, likely as a result of large variance in the
groups, none of these relationships approached
statistical significance. Although the correlation
coefficients were small revealing weak relationships,
we did find significantly negative correlations
between body height and test duration in every
group (male varsity, male non-varsity, female nonvarsity) with the exception of female varsity
athletes. There were also similar weak relationships
between body mass and test duration for every
group. These findings support the possibility that
body height, mass, and sex are determining factors
in predicting time to fatigue. Additionally, noting
the relationship between body size and lower test
durations, because both athletes and males were on
average taller and heavier than non-athletes and
females respectively, separate percentiles for
athletic status by gender are appropriate.
As expected, both physical activity and
athletic status were significantly related to test
duration where it was found that with increasing
physical activity levels would produce increases in
muscular endurance of the abdominal core (Figure
1). Again, a between-groups sex-specific difference
was not found in the within-groups categories of
activity level or across athletic status.
One
interesting finding was seen in the dose-response
relationship of physical activity and test duration.
100
With all physical activity levels there was an
increase in mean test duration time but this only
reached a level of significance at higher weekly
activity levels. As compared to those that were
never or rarely physically active, only statistically
higher test durations were seen in the groups active
more than three times per week. This suggests a
threshold of activity that is necessary to increase
abdominal endurance where being active less than
three times per week would not be expected to
significantly increase fatigue onset. Secondly, the
highest activity levels are significantly related to the
longest test durations where those participants
reporting activity more than five days per week had
test durations averaging 150 seconds which was
56% longer than those active 3-5 times per week.
The concluding suggestion is that for expected
improvements to occur in abdominal endurance, at
least three times per week would need to be
prescribed and programming over five days per
week would be expected to produce the most
benefit.
Reason for test termination was found to be
significantly related to test duration but no
differences were seen between the sexes.
Interestingly, abdominals, posture, legs, and other
were all statistically similar whereas the back and
arms were both significantly higher than the
abdominal groups. One explanation for this might
be that if the abdominals were undertrained, they
would be the weak-link in the abdominal plank and
therefore produce discomfort due to fatigue earlier
than if they were adequately trained. Adequately
trained abdominals would result in participants
reporting other muscular regions as being the
primary reason for test termination. Shorter tests
therefore would be most likely to be terminated
because of abdominal discomfort and therefore
serve as a valuable diagnostic in determination of
priority where additional abdominal training might
be prescribed to decrease the chances of injury of
the core. An example of this is that our participants
citing abdominal discomfort as a primary reason
for termination displayed a mean of 87 s with a
standard deviation of 49 s. This is in accordance
with previous research suggesting athletes should
maintain plank positions for at least 60 s (Bliss and
Norms for an Isometric Muscle Endurance Test
Teeple, 2005). This finding supports the efficacy of
the plank test for abdominal endurance assessment
by producing short test durations and thus low
percentile rankings as a result of failure of the
abdominals.
The investigation into anthropometrics was
included in the hypothesis that higher body mass
and height might produce a greater challenge to
plank success because greater torque would need to
be generated and sustained over time in order to
maintain static position resulting from these
variables. This was confirmed as statistical
differences were seen in mean time to fatigue by sex
and separately by athletic status. However, this
effect was lost when examined by sex and athletic
status concurrently. Males compared to females and
athletes compared to non-athletes in this study
were both statistically taller and heavier, which
suggests body size differences could reduce the
observed magnitude of real differences in time to
fatigue when compared to females or non-athletes.
Future work specifically examining the interaction
effects of body size, athletic status, and sex is
suggested to elucidate the relative contributions of
each to time and fatigue in the forearm plank test
Our data reflect an attempt to produce
valid norms for abdominal core endurance in
college-aged sex and athletic status. Future work
should expand the age of these normative
percentile rankings to encompass different age
groups which include children, adolescents, as well
as a range of adult groups. Secondly, because of
training specificity with regard to muscular activity,
energetics, and the nature of the kinetic chain,
greater attention should be paid to different athletic
groups. Third, the physical activity data presented
herein were retrospective and thus cross-sectional
in nature. This leaves a need for an intervention
investigation into the prospective effects of physical
activity and core-specific training in the ability to
influence abdominal endurance scores as produced
with plank tests. A final, albeit more complicated
but much more crucial area of future work should
investigate the link between abdominal endurance
and prospective injury incidence.
Acknowledgement
Anthony Quinn, Kelly Potteiger, Andrew Lundgren.
Journal of Human Kinetics volume 40/2014
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by Strand S.L. et al.
101
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Corresponding author:
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Journal of Human Kinetics volume 40/2014
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